1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging scheme for obtaining medical diagnostic data by measuring the density distribution of specific nuclei within a living body non-invasively by utilizing the nuclear magnetic resonance phenomenon.
2. Description of the Background Art
As already well known, the nuclear magnetic resonance imaging method (abbreviated hereafter as MRI) is a method for obtaining the microscopic physical and chemical data of the molecules by utilizing the nuclear magnetic resonance phenomenon in which, when the nuclei having the characteristic spin and magnetic moment associated with it are placed in a uniform static magnetic field of the strength H.sub.0, the nuclei resonantly absorb the radio frequency magnetic field which is rotating at the angular speed of .omega..sub.0 =.gamma.H.sub.0, where .gamma. is a gyromagnetic ratio which is a characteristic constant for a specific nucleus type, on a plane perpendicular to the direction of the static magnetic field.
In this nuclear magnetic resonance imaging method, there are several known schemes for imaging the spatial distribution of the specific nuclei within the living body to be examined, such as hydrogen nuclei in the water and the fat for example, including the zeugmatography scheme of Lauterbur, the Fourier scheme of Kumar, Welti, Ernst, et al., and the spin warp scheme of Hitchison which can be considered as the modification of the Fourier scheme. In addition, there are several known schemes for carrying out the image re-construction at high speed, including the high speed Fourier scheme and the echo planar scheme of Mansfield.
Now, in general, when the magnetic field B changes in time, its rate of change dB/dt induces the eddy currents within the living body placed in that changing magnetic field. In this regard, in the ultra high speed imaging schemes mentioned above, the stronger than usual reading gradient magnetic field is applied repeatedly while switching, i.e., inverting its polarity, at high speed, so that the eddy currents can be induced in the living body under the examination using the ultra high speed imaging scheme. The typical exemplary waveforms of the gradient magnetic fields B used in the ultra high speed imaging schemes are shown in FIGS. 1A and 1B, along with their corresponding dB/dt waveform, where FIG. 1A is the trapezoidal gradient magnetic field and FIG. 1B is the sinusoidal gradient magnetic field.
In particular, in a case of using the gradient magnetic field which is both strong as well as switching at high speed as in the ultra high speed imaging scheme, the induced eddy current density can exceed the minimum stimulation level of the nerve of the living body, and can be sensed by the living body as the nerve stimulation. Here, the induced eddy current density J (A/m.sup.2) can be expressed by the following equation (1): EQU J=.sigma..multidot.r/2.multidot.dB/dt (1)
where r (m) is a radius of the induced eddy current flow, and .sigma. (sec/m) is a conductivity of the living body. This expression has been disclosed in T. F. Budinger et al., "Health Effects of in Vivo Nuclear Magnetic Resonance", Biomedical Magnetic Resonance (T. L. James et al. editors), pp. 421-437, 1984.
In this regard, there are reports indicating that the nerve stimulation can be caused by the current density in a range of J=1 to 4 (A/m.sup.2), so that when the field change dB/dt is capable of causing the induced eddy current density in this range, the nerve stimulation can be caused in the living body. (See: D. McRobbie, et al., Clinical Physics for Physiological Measurement. Vol. 5, p. 67, 1984; and H. Yamagata et al., "Evaluation of dB/dt Thresholds for Nerve Stimulation Elicited by Trapezoidal and Sinusoidal Gradient Fields in Echo-Planar Imaging" in "Proceedings of 10th Annual Meeting of Magnetic Resonance in Medicine, Works in Progress, San Francisco, 1991", p. 1277.)
In further detail, the actual field change dB/dt generated in the MRI will be explained with reference to FIG. 2. Namely, the gradient magnetic field Gx normally used for the imaging in the MRI causes a magnetic field component xGx in the Z direction as shown in FIG. 2. On the other hand, according to the electromagnetic field theory of Maxwell, it is known that there exists a cross magnetic field component zGx in the X direction perpendicular to the gradient magnetic field Gx. In this case, as the patient's body is lying along the Z direction, the patient's body extends farther in the Z direction than in the X direction, so that the cross magnetic field component zGx on the patient's body is larger than the magnetic field component xGx on the patient's body.
Consequently, the possibility of causing the nerve stimulation is greater for the cross magnetic field component zGx than the magnetic field component xGx. In other words, when the gradient magnetic field is centered at the chest portion in order to image the chest portion, the nerve stimulation is more likely caused at the head portion and the waist portion which are farther distanced from the center of the gradient magnetic field.
Moreover, the eddy currents become greater for the larger radius r as can be seen in the above equation (1), so that even for the same field change dB/dt, the eddy current density J becomes greater at the portion at which the eddy current flow radius r is relatively larger, i.e., the cross sectional area through which the eddy currents are flowing is larger. Consequently, in the imaging of the chest portion, the nerve stimulation is more likely caused at the head portion or the waist portion which has the larger cross sectional area.
In fact, there is a report that when the sinusoidal gradient magnetic field is used as Gx, the nerve stimulation has been sensed at the middle of the forehead for dB/dt=61 Tesla/sec. (See, M. S. Cohen, et al., "Sensory Stimulation by Time-varying Magnetic Fields", Magnetic Resonance in Medicine, Vol. 114, No. 3, pp. 409-414, 1990.)
Thus, in the conventional MRI apparatus, there has been a problem of the occurrence of the nerve stimulation due to the eddy currents, particularly at the head and waist portions, which can be at least uncomfortable for the patient even if it is not painful.
On the other hand, the ultra high speed imaging scheme has the advantage of the short imaging time as well as the disadvantage that a high resolution is difficult to obtain in the one shot type operation due to the limitations on the hardware. To remedy this disadvantage, there has been a proposition for division scan scheme in which one frame of the image is obtained by a plurality of scans, by sacrificing the imaging time somewhat.
However, such ultra high speed imaging scheme and division scan scheme still basically require the reading gradient magnetic field to be switched at high speed, so that the influence of the magnetic field inhomogeneity appears oppositely at the even turns and the odd turns. Consequently, when the NMR signals acquired by these schemes are reconstructed straightforwardly, the artifact called N/2 artifact in which the ghost appears at positions distanced by a half of a size of the imaging region from the actual image can be produced on the re-constructed MR image, to degrade the image quality considerably.
In order to avoid this N/2 artifact, it has conventionally been necessary to carry out a complicated procedure for measuring and correcting the homogeneity of the static magnetic field.
In addition, in a case a moving object such as the blood is present in the imaging region, it has been possible in the ordinary conventional imaging schemes to suppress the artifact due to the motion of the moving object by carrying out the phase re-focusing, i.e., the addition of the phase compensation gradient magnetic fields, in three directions into which the gradient magnetic fields are applied. However, in a case of the ultra high speed imaging scheme or division scan scheme, the reading gradient magnetic field switching at high speed is used, so that the phase shift component due to the motion speed cannot be set equal to zero for both the even turns and the odd turns, so that it has been difficult to suppress the artifact due to the motion in the gradient magnetic field direction.
Moreover, in the ultra high speed imaging scheme and the division scan scheme, the phase encoding gradient magnetic field is much weaker than the reading gradient magnetic field, so that there has been a problem that the so called chemical artifact in which the fat component is displaced in the phase encoding direction can be produced on the re-constructed MR image.